A DC/DC converter (hereinafter simply referred to as “converter”) that converts direct current (DC) voltage to desired DC voltage by raising or lowering the voltage has been used in various fields. A converter outputs required voltage by switching current flowing through an inductor. In order to increase output current while reducing ripple of the output current and to miniaturize the device, a multi-phase (multiple phase) converter has been employed that includes multiple converters that are connected in parallel and that operate in different phases.
For example, Patent Document 1 (Japanese Patent Application Publication Laid-Open No. 2005-224058) describes a step-up DC/DC converter that alternately applies excitation current to the first and second coils using first and second coils wound in opposite directions around the same core as well as a pair of switches, to generate current at the second and first coils in directions canceling out the magnetic fluxes of the core and to output current from the second and first coils accumulated by a diode. This DC/DC converter may have a reduced core size, since the magnetization of the core by the first and second coils are canceled out to suppress magnetic saturation of the core.
It has been known that, in a so-called interleaved DC/DC converter using the technique described in Patent Document 1, a coupling coefficient between the first and second coils is closely related to the ripple current in the first and second coils. In view of this, Patent Document 2 (Japanese Patent Application Publication Laid-Open No. 2015-231311) discloses a technique of reducing ripple current by setting a coupling coefficient k between inductors to be within the range of −0.4<k≤−0.1 in the case where the current flowing through two inductors corresponding to the first and second coils is switched at a duty ratio in the range from 5% to 40%, compared to the case where k is 0.